Henri. I don't know this for sure or if it applies to every machinists level but I was taught that a machinists level had a shallow v notch in the base so that when you were leveling a shaft the notch would ensure that you were parralel to the shaft. Machinist levels as I know them were aslo very precise when I was working we had levels that varied from 1/2 a thou per inch to 5 thou per inch and were any where from 6" long to 18" long. Think I still have a few hiding out in the shop.We had a few different brands but I think Starrett was the most common.

Interesting technical explanation. It make sense.My father never used this level. He inherited it from his brother, a fortress artillery officer in the first world war.Perhaps the level was used by him.

Maybe I can just call it old "level" from now on and not "machinist's level".

Machinist's levels will generally be very sensitive, detecting very small changes in level. Set it on your bench and place thin paper - card stock or printer paper - under it until the bubble is centered. It's handy to put down a piece of printer paper first, and mark around the level, for reasons I'll explain in the next paragraph.

While you've got the bubble centered, pick up the level and turn it end for end, carefully setting it down in the exact same place (that's why you marked around it in the first step). If the bubble's still centered, it is in fact a level; if not, it's a pretty little piece of metal with a bubble in it; making it into a level would require getting a replacement vial (the part with the liquid and bubble in it) and carefully replacing the old one, installing the new one so it's accurately level, not an easy job.

Now add one more piece of paper, and see how much the bubble moves. If it takes several pieces of paper before you see much movement, it's probably just a handy little level; if the bubble moves quickly, it's likely a machinist's level.